The Australia Act 1986 is an Act to bring constitutional arrangements affecting the Commonwealth and the States into conformity with the status of the Commonwealth of Australia as a sovereign, independent and federal nation. The Australia Act 1986 (Act No. 142, 1985) was assented to on 4 December 1985 and came into operation on 3 March 1986 at 5.00 A.M. Greenwich Mean Time[33] when it was proclaimed by Queen Elizabeth II, who travelled to Canberra for that purpose to make it clear that she was acting in her capacity as Queen of Australia. In addition to this Australia Act 1986 an Australia Act 1986 , in substantially identical terms, was enacted by the United Kingdom Parliament (1986 Chapter 2) pursuant to a request made and consent given by the Parliament and Government of the Commonwealth in the Australia (Request and Consent) Act 1985 and with the concurrence of all States of Australia (see the Australia Acts Request Act 1985 of each State).[34] The suffixes (Cth) and (UK) are often added to the titles of the respective Acts so as to differentiate one from the other. The UK Act has its source in the Statute of Westminster (s4, then available), the Cth Act in the Constitution (s.51 (xxxviii)). The former was passed in case the latter was found to be invalid.[35]

By joint action of all the Parliaments of Australia and the United Kingdom, the legislative, executive and judicial institutions of the United Kingdom ceased to have any power, responsibility or jurisdiction in respect of Australian affairs.[36] Accordingly, the United Kingdom Parliament has implicitly 'abandoned' its Constitution Act of 1900 to the Commonwealth, States and their Territories for their local handling.[37]

A High Court decision of 1989 has also put at rest any doubts as to the validity of the Australia Act 1986 (Cth). [38]